Why doesn’t bleach get rid of drain flies?

Because the flies are not in the water. Drain flies lay their eggs in a thin layer of slime that coats the inside of the pipe, the trap and the overflow. Bleach, boiling water and most unblockers rush past that layer on their way down, so the eggs and larvae in it carry on growing. That is why they are back an hour after you clean: the next ones were already in the pipe.

The fix is not a stronger liquid. It is getting the film off the pipe wall, keeping it off, and letting the flies already out live out their short lives.

First, make sure it is the drain

1. Look at one that has landed. Drain flies are small, grey or brown and fuzzy, with wings like a tiny moth’s. They sit flat on tiles and walls and flutter more than fly. If they have red eyes and hang round fruit and the bin, see fruit flies or drain flies first.
2. Do the tape test. Before bed, lay a strip of clear tape over the plughole, sticky side down, leaving a small gap for air. Flies stuck to the underside in the morning mean that drain is breeding them.
3. Test every drain, one at a time. Kitchen sink, basin, bath, shower, both overflows, a floor drain, the washing machine outlet. Often only one is the source, and it is not always the one you suspect.

Clear the film, step by step

1. Take out what comes out. Plug, pop-up stopper, hair catcher. Pull the hair and gunk out of the top of the pipe.
2. Scrub the inside of the pipe. A long, narrow, stiff drain brush, worked up and down the pipe and into the overflow channel. This is the step most people skip, and the one that matters most.
3. Follow with an enzyme or foaming drain cleaner. These are made to break down the organic build-up the larvae live in, and foam clings to the pipe wall where liquids run past. Use it last thing at night so it sits undisturbed, and follow the label.
4. Repeat as the label directs on every drain the tape test flagged. One clean rarely gets it all.
5. Find the hidden ones. Bath and basin overflows, the trap under a shower tray, a sink in a spare bathroom, a utility room floor drain. A drain that is rarely used can dry out and build slime, so run water through it once a week.

A newly fitted whirlpool or jetted bath has its own pipework that holds water. Run its cleaning cycle as the maker’s instructions say, because flies can breed in there too.

Why bleach and boiling water let you down

Most people with drain flies have tried bleach, boiling water, unblocker, vinegar and baking soda, often in the same week. It feels thorough. But anything that flows past in a few seconds does not shift a film stuck to the pipe wall, and the larvae sit right inside it. A fogger in the bathroom does not reach them either.

The bigger risk is mixing. Pouring several products down the same drain, one after another, can do more harm than the flies ever will.

Safety: never mix bleach with other cleaners, vinegar or drain products, because some mixes give off harmful gas. Use one product at a time, run plenty of water between them, open a window, wear gloves, and keep every bottle where children and pets cannot reach it.

While you work on the drains, a sticky trap near the plughole catches adults and shows you whether numbers are falling. It will not end the problem on its own, because new flies keep hatching until the film is gone.

How long until they go?

Even after a good scrub you will probably see a few for some days. Those are adults that had already hatched. A week or two after the drains are properly clean is a fair window for the last of them.

Keep doing the tape test. Clean tape morning after morning means the drain is no longer breeding them. Then keep it that way: a hair catcher, a brush through the pipe now and then, and a plug in at night if flies still turn up.

When it is not the drains you can see

If the tape stays clean on every drain and the flies keep coming, the source is somewhere out of reach: a slow drain building fresh slime, a leak under a bath or shower tray, or a cracked pipe under the floor. That is a job for a plumber, not a pest controller. Our answer on flies and blocked drains covers what to check and what to tell them. Gaps where pipes come through the wall can also let flies in from a damp void, so it is worth sealing those too.